It's a matter of code readability. "when" and "unless" are expressions too, but they look the best when they're used instead of "if" for single-branched conditionals when you don't care about return value.

Yes - from what I recall it definitely helped to make things clearer to use "if" and "when" appropriately.

The choice exists only if you have a single form to evaluate subject to the condition. If there are two or more, of course this is imperative programming and (when condition form1 form2 ...) recommends itself over (if condition (progn form1 form2 ...)). If there is only form1, either one will do, so then it's down to a decision based on whether form1 returns a useful value and/or performs a side effect.